Move base into ownmacro/slot, make other urls relative

There is now only one absolute url left. Unfortunately base tags
cannot take relative paths. We can hack this using JS.
Make all paths relative, drop base tag

For this demo, it is okay that we operate <base/>less. The slot is still
there and will be filled up remix.
Clean up our settings file a bit.
Merge branch 'oerpub-ticket372-pull2' into testing

Conflicts:
	oerpub/index.html